Aqueous flexographic and gravure inks are widely used in the industry for a number of printing purposes, including printing on plastic packaging films. Support resins employed as vehicles for such water-based flexographic and gravure ink formulations must exhibit properties such as solubility in water, wet adhesion to polypropylene film, hydrolytic stability, high melting point, and good pigment wetting.
Alcohol-soluble polyamide resins have been used extensively in inks for packaging and are commercially available. These polyamides are made from dimerized fatty acids and various polyamines such as ethylene diamine or hexamethylene diamine.
The use of such polyamide resins in ink compositions is described in Floyd, D. E., Polyamide Resins, Reinhold Publishing Co., New York, 1958 and in the Encyclopedia of Polymer Science and Technology, Interscience Publishers, John Wiley Sons, Volume 10, New York, 1969. A typical commercial product is GAX-340 manufactured by Henkel.
For water-based ink, a water-soluble support resin such as acrylic resin or a conventional soluble maleic resin may be used. Acrylic support resins have good film properties, but lack adhesion to polyolefin films.
Conventional soluble maleic resins, which contain half-esters, are subject to a certain degree of hydrolytic instability under alkaline conditions. They also lack film toughness.
Water-based ink compositions are taught in U.S. Pat. No. 4,963,188 to be prepared by free radical polymerization of rosin and maleic anhydride. Modification of the polymer with an alcohol or an amine prior to utilization in preparing the ink composition is disclosed.
Recently, water-soluble resins having qualities of adhesion and wettability for use in packaging ink compositions were developed as the reaction products of rosin modified by Diels-Alder reaction with an .alpha.,.beta. unsaturated acid and a compound containing two secondary amine groups, including commercially available piperazine and N,N'-dimethylethylene diamine. These resins are described in U.S. Pat. No. 5,066,331.
Such resins were developed primarily for use in water/isopropanol-based inks for printing on film, where they perform quite satisfactorily. When using these resins in all-water systems (i.e., no alcohol), it was found that the resin solutions had relatively limited shelf stability. In many cases the solutions gel upon standing for two or three days.
U.S. Pat. No. 5,152,832 (which is hereby incorporated by reference) corrected this gelling problem by teaching the use of a resin derived from the reaction products obtained by reacting a modified rosin with a compound containing two secondary amine groups, and further modifying said reaction products with a polyol (such as diethylene glycol).
However, a major problem develops when one attempts to utilize the resins taught in U.S. Pat. No. 5,152,832 as support resins for emulsion polymerizations. These resins form excessive grit whenever they are employed in this manner. This characteristic renders these resins totally unsuitable for use as support resins for making acrylic latexes.
The present invention solves this problem by teaching acrylic latex compositions formed by reacting a monomer with a support resin derived from the reaction products obtained by reacting a modified rosin with an alkanolamine compound containing at least one secondary amine and at least one hydroxyl group. Thus, by requiring that the amine and hydroxyl groups be contained in the same molecule (rather than in two separate ones), the present invention utilizes a support resin which substantially differs from resin taught in U.S. Pat. No. 5,152,832.
Therefore, it is an object of this invention to provide improved acrylic latex compositions which may be used to prepare water-based flexographic gravure ink formulations.